Automated solution-phase synthesis of oligosaccharides via iterative electrochemical assembly of thioglycosides

Overview of the current procedures in synthesis of heparin saccharides

Natural heparin, a glycosaminoglycan consisting of repeating hexuronic acid and glucosamine linked by 1 → 4 glycosidic bonds, is the most widely used anticoagulant. To subvert the dependence on animal sourced heparin, alternative methods to produce heparin saccharides, i.e., either heterogenous sugar chains similar to natural heparin, or structurally defined oligosaccharides, are becoming hot subjects. Although the success by chemical synthesis of the pentasaccharide, fondaparinux, encourages to proceed through a chemical approach generating homogenous product, synthesizing larger oligos is still cumbersome and beyond reach so far. Alternatively, the chemoenzymatic pathway exhibited exquisite stereoselectivity of glycosylation and regioselectivity of modification, with the advantage to skip the tedious protection steps unavoidable in chemical synthesis. However, to a scale of drug production needed today is still not in sight. In comparison, a procedure of de novo biosynthesis in an organism could be an ultimate goal. The main purpose of this review is to summarize the current available/developing strategies and techniques, which is expected to provide a comprehensive picture for production of heparin saccharides to replenish or eventually to replace the animal derived products. In chemical and chemoenzymatic approaches, the methodologies are discussed according to the synthesis procedures: building block preparation, chain elongation, and backbone modification.

Electrochemical Glycosylation via Halogen-Atom-Transfer for C-Glycoside Assembly

Glycosyl donor activation emerged as an enabling technology for anomeric functionalization, but aimed primarily at O-glycosylation. In contrast, we herein disclose mechanistically distinct electrochemical glycosyl bromide donor activations via halogen-atom transfer and anomeric C-glycosylation. The anomeric radical addition to alkenes led to C-alkyl glycoside synthesis under precious metal-free reaction conditions from readily available glycosyl bromides. The robustness of our e-XAT strategy was further mirrored by C-aryl and C-acyl glycosides assembly through nickela-electrocatalysis. Our approach provides an orthogonal strategy for glycosyl donor activation with expedient scope, hence representing a general method for direct C-glycosides assembly.

HPLC-Based Automated Synthesis and Purification of Carbohydrates

Reported herein is a new HPLC-based automated synthesizer (HPLC-A) capable of a temperature-controlled synthesis and purification of carbohydrates. The developed platform allows to perform various protecting group manipulations as well as the synthesis of O- and N-glycosides. A fully automated synthesis and purification was showcased in application to different carbohydrate derivatives including glycosides, oligosaccharides, glycopeptides, glycolipids, and nucleosides.

Synthesis of cyclic β-1,6-oligosaccharides from glucosamine monomers by electrochemical polyglycosylation

The synthesis of protected precursors of cyclic β-1,6-oligoglucosamines from thioglycosides as monomers is performed by electrochemical polyglycosylation. The monomer with a 2,3-oxazolidinone protecting group afforded the cyclic disaccharide exclusively. Cyclic oligosaccharides up to the trisaccharide were obtained using the monomer with a 2-azido-2-deoxy group.

Recent chemical synthesis and immunological evaluation of glycans related to bacterial lipopolysaccharides

O-Antigens and core oligosaccharides from bacterial lipopolysaccharides (LPS) are often structurally unique and immunologically active, have become attractive targets in the development of antibacterial vaccines. Structurally well-defined and pure oligosaccharides can be used in identifying protective epitopes of the carbohydrate antigens, which is important for the design of an effective vaccine. Here, the recent progress on chemical synthesis and immunological evaluation of glycans related to O-antigens and core oligosaccharides from bacterial LPS are summarized.

Electrochemical synthesis of the protected cyclic (1,3;1,6)-β-glucan dodecasaccharide

Automated electrochemical assembly is an electrochemical method to synthesise middle-sized molecules, including linear oligosaccharides, and some linear oligosaccharides can be electrochemically converted into the corresponding cyclic oligosaccharides effectively. In this study, the target cyclic oligosaccharide is a protected cyclic (1,3;1,6)-β-glucan dodecasaccharide, which consists of two types of glucose trisaccharides with β-(1,3)- and β-(1,6)-glycosidic linkages. The formation of the protected cyclic dodecasaccharide was confirmed by the electrochemical one-pot dimerisation-cyclisation of the semi-circular hexasaccharide. The yield of the protected cyclic dodecasaccharide was improved by using a stepwise synthesis via the linear dodecasaccharide.

Towards one-pot selective synthesis of cyclic oligosaccharides

In this chapter are described electrochemical routes to cyclic oligosaccharides. While automated electrochemical methods have been used to prepare linear oligosaccharides, their conversion to cyclic oligosaccharides proved to be a complex process. The concept of polyglycosylation offers an interesting alternative, and the process which has been developed is that of a one-pot electrochemical polyglycosylation-isomerization-cyclization (ePIC) process.

VaporSPOT: Parallel Synthesis of Oligosaccharides on Membranes

Automated chemical synthesis has revolutionized synthetic access to biopolymers in terms of simplicity and speed. While automated oligosaccharide synthesis has become faster and more versatile, the parallel synthesis of oligosaccharides is not yet possible. Here, a chemical vapor glycosylation strategy (VaporSPOT) is described that enables the simultaneous synthesis of oligosaccharides on a cellulose membrane solid support. Different linkers allow for flexible and straightforward cleavage, purification, and characterization of the target oligosaccharides. This method is the basis for the development of parallel automated glycan synthesis platforms.

Synthesis of protected precursors of chitin oligosaccharides by electrochemical polyglycosylation of thioglycosides

The synthesis of protected precursors of chitin oligosaccharides by electrochemical polyglycosylation of thioglycosides as monomer is described. Oligosaccharides up to the hexasaccharide were synthesized under optimized reaction conditions. Further, a modified method enabled the synthesis of oligosaccharides up to the octasaccharide by repeating electrolysis with additional monomers. The mechanism of the electrochemical polyglycosylation is also discussed, based on the oxidation potential of the monomer and oligosaccharides.

HPLC-Based Automated Synthesis of Glycans in Solution

As the 21st century unfolds with rapid changes, new challenges in research and development emerge. These new challenges prompted us to repurpose our HPLC-A platform that was previously used in solid phase glycan synthesis to a solution phase batch synthesis described herein. The modular character of HPLC allows for implementing new attachments. To enable sequential synthesis of multiple oligosaccharides with the single press of a button, we supplemented our system with a four-way split valve and an automated fraction collector. This enabled the operator to load all reagents and all reactants in the autosampler, press the button to start the repetitive automation sequence, leave the lab, and upon return find products of multiple reactions ready for purification, analysis, and subsequent application.

Synthesis of cyclic α-1,4-oligo-N-acetylglucosamine 'cyclokasaodorin' via a one-pot electrochemical polyglycosylation-isomerization-cyclization process

Electrochemical synthesis of unnatural cyclic oligosaccharides composed of N-acetylglucosamine with α-1,4-glycosidic linkages has been accomplished. A thioglycoside monomer equipped with the 2,3-oxazolidinone protecting group was used to prepare linear oligosaccharides by electrochemical polyglycosylation. In the same pot, isomerization of the linear oligosaccharides and intramolecular electrochemical glycosylation for cyclization were also conducted sequentially to obtain the precursor of the cyclic α-1,4-oligo-N-acetylglucosamine 'cyclokasaodorin'.

Anodic Reactivity of Alkyl S-Glucosides

A voltammetric study of a series of alkyl and aryl S-glucosides unveiled the reactivity patterns of alkyl S-glucosides toward anodic oxidation and found noteworthy differences with the trends followed by aryl derivatives. The oxidation potential of alkyl S-glucosides, estimated herein from square-wave voltammetry peak potentials (E_p), depends on the steric properties of the aglycone. Glucosides substituted with bulky groups exhibit E_p values at voltages more positive than the values of those carrying small aglycones. This relationship, observed in all analyzed alkyl series, is evidenced by good linear correlations between E_p and Taft's steric parameters (E_S) of the respective alkyl substituents. Moreover, the role of the aglycone's steric properties as a primary reactivity modulator is backed by poor correlations between E_p and the radical stabilization energies (RSEs) of the aglycone-derived thiyl radicals (RS•). In contrast, aryl glucosides' E_p values exhibit excellent correlations with the aryl substituents' Hammett parameters (σ+) and the ArS• RSEs, evidencing the inherent stability of the reactive radical intermediate as the primary factor controlling aryl glucoside's electrochemical reactivity. The reactivity differences between alkyl and aryl S-glucosides also extend to the protective group's effect on E_p. Alkyl S-glucosides' reactivity proved to be more sensitive to protective group exchange.

Small tools for sweet challenges: advances in microfluidic technologies for glycan synthesis

Glycans, including oligosaccharides and glycoconjugates, play an integral role in modulating the biological functions of macromolecules. Many physiological and pathological processes are mediated by interactions between glycans, which has led to the use of glycans as biosensors for pathogen and biomarker detection. Elucidating the relationship between glycan structure and biological function is critical for advancing our understanding of the impact glycans have on human health and disease and for expanding the repertoire of glycans available for bioanalysis, especially for diagnostics. Such efforts have been limited by the difficulty in obtaining sufficient quantities of homogenous glycan samples needed to resolve the exact relationships between glycan structure and their structural or modulatory functions on a given glycoconjugate. Synthetic strategies offer a viable route for overcoming these technical hurdles. In recent years, microfluidics have emerged as powerful tools for realizing high-throughput and reproducible syntheses of homogenous glycans for the potential use in functional studies. This critical review provides readers with an overview of the microfluidic technologies that have been developed for chemical and enzymatic glycan synthesis. The advantages and limitations associated with using microreactor platforms to improve the scalability, productivity, and selectivity of glycosylation reactions will be discussed, as well as suggested future work that can address certain pitfalls.

Photolabile 2-(2-Nitrophenyl)-propyloxycarbonyl (NPPOC) for Stereoselective Glycosylation and Its Application in Consecutive Assembly of Oligosaccharides

A photolabile protecting group (PPG) 2-(2-nitrophenyl)-propyloxycarbonyl (NPPOC) was explored in glycosylation and applied in the consecutive synthesis of oligosaccharides. NPPOC displays a strong neighboring group participation (NGP) effect to facilitate the construction of 1,2-trans glycosides in excellent yield. Notably, NPPOC could be efficiently removed by photolysis, and the deprotection conditions are friendly to typical protecting groups. A branched and asymmetric oligomannose Man6 was rapidly prepared, and the consecutive assembly of oligosaccharides without intermediate purification was further investigated owing to the compatibility conditions between NPPPOC's photolysis and glycosylation.

Silver-assisted gold-catalyzed solid phase synthesis of linear and branched oligosaccharides

Unlike solid phase synthesis of peptides, synthesis of oligosaccharides by solid phase methods is lagging behind owing to inherent challenges faced while executing glycosidations. In this communication, silver-assisted gold-catalyzed glycosidations are found to be excellent for solid phase oligosaccharide synthesis. Glycosidations under catalytic conditions, one time coupling with four equivalents of donor, reactions in less than 30 min at 25 °C and on-resin deprotection of silyl and benzoate protecting groups are the salient features. Photocleaved glycans possess a protected amino functionality that can be utilized for bioconjugation. The versatility of this approach is established by synthesizing linear and branched pentaarabinofuranosides.

Electrocatalysis as a key strategy for the total synthesis of natural products

Electrochemical strategies have been a powerful approach for the synthesis of valuable intermediates, in particular heterocyclic motifs. Because of the mild nature, a wide range of nonclassical bond disconnections have been achieved via in situ-generated radical intermediates in a highly efficient manner. In particular, anodic electrochemical oxidative strategies have been utilized for the total synthesis of many structurally intriguing natural products. In this review article, we have discussed a number of total syntheses of structurally intriguing alkaloids and terpenoids in which electrochemical processes play an important role as a key methodology.

Glycan Assembly Strategy: From Concept to Application

Glycans have been hot topics in recent years due to their exhibition of numerous biological activities. However, the heterogeneity of their natural source and the complexity of their chemical synthesis impede the progress in their biological research. Thus, the development of glycan assembly strategies to acquire plenty of structurally well-defined glycans is an important issue in carbohydrate chemistry. In this review, the latest advances in glycan assembly strategies from concepts to their applications in carbohydrate synthesis, including chemical and enzymatic/chemo-enzymatic approaches, as well as solution-phase and solid-phase/tag-assisted synthesis, are summarized. Furthermore, the automated glycan assembly techniques are also outlined.

Progress and challenges in the synthesis of sequence controlled polysaccharides

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure-properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

Electrochemical Assembly for Synthesis of Middle-Sized Organic Molecules

Electrochemical methods offer a powerful, reliable, and environmentally benign approach for the synthesis of small organic molecules, and such methods are useful not only for the transformation of small molecules, but also for the preparation of oligomers and polymers. Electrochemical assembly is a concept that allows structurally well-defined middle-sized organic molecules to be synthesized by applying electrochemical methods. The preparation of dendrimers, dendronized polymers, and oligosaccharides are introduced as examples of such an approach. Automated electrochemical assembly of oligosaccharides is also demonstrated using the electrochemical synthesizer developed by our group.

Synthetic Carbohydrate Chemistry and Translational Medicine

The importance of post-translational glycosylation in protein structure and function has gained significant clinical relevance recently. The latest developments in glycobiology, glycochemistry, and glycoproteomics have made the field more manageable and relevant to disease progression and immune-response signaling. Here, we summarize the current progress in glycoscience, including the new methodologies that have led to the introduction of programmable and automatic as well as large-scale enzymatic synthesis, and the development of glycan array, glycosylation probes, and inhibitors of carbohydrate-associated enzymes or receptors. These novel methodologies and tools have facilitated our understanding of the significance of glycosylation and development of carbohydrate-derived medicines that bring the field to the next level of scientific and medical significance.

Review: Advances in preparation of chitooligosaccharides with heterogeneous sequences and their bioactivity

Chitooligosaccharides has attracted increasing attention due to their diverse bioactivities and potential application. Previous studies on the bioactivity of chitooligosaccharides were mostly carried out using a mixture. The structure-function relationship of chitooligosaccharides is not clear. Recently, it is confirmed that chitooligosaccharides with different degrees of polymerization play different roles in many bioactivities. However, heterogeneous chitooligosaccharides with a single degree of polymerization is still a mixture of many uncertain sequences and it is difficult to determine which structure is responsible for biological effects. Therefore, an interesting and challenging field of studying chitooligosaccharides with heterogeneous sequences has emerged. Herein, we reviewed the current methods for preparing heterogeneous chitooligosaccharides, including chemical synthesis, separation techniques and enzymatic methods. Advances in the bioactivities of chitooligosaccharides with heterogeneous sequences are also reviewed.

Recent Progress in Chemo-Enzymatic Methods for the Synthesis of N-Glycans

Asparagine (N)-linked glycosylation is one of the most common co- and post-translational modifications of both intra- and extracellularly distributing proteins, which directly affects their biological functions, such as protein folding, stability and intercellular traffic. Production of the structural well-defined homogeneous N-glycans contributes to comprehensive investigation of their biological roles and molecular basis. Among the various methods, chemo-enzymatic approach serves as an alternative to chemical synthesis, providing high stereoselectivity and economic efficiency. This review summarizes some recent advances in the chemo-enzymatic methods for the production of N-glycans, including the preparation of substrates and sugar donors, and the progress in the glycosyltransferases characterization which leads to the diversity of N-glycan synthesis. We discuss the bottle-neck and new opportunities in exploiting the chemo-enzymatic synthesis of N-glycans based on our research experiences. In addition, downstream applications of the constructed N-glycans, such as automation devices and homogeneous glycoproteins synthesis are also described.

Stereoselective Electro-2-deoxyglycosylation from Glycals

We report a novel and highly stereoselective electro-2-deoxyglycosylation from glycals. This method features excellent stereoselectivity, scope, and functional-group tolerance. This process can also be applied to the modification of a wide range of natural products and drugs. Furthermore, a scalable synthesis of glycosylated podophyllotoxin and a one-pot trisaccharide synthesis through iterative electroglycosylations were achieved.

Total synthesis of Myc-IV(C16:0, S) via automated electrochemical assembly

Total synthesis of Myc-IV(C16:0, S) via automated electrochemical assembly has been accomplished. This tetrasaccharide has been studied as a symbiotic signal molecule of Arbuscular Mycorrhiza fungi. We have achieved stereoselective synthesis of a disaccharide building block using the mixed-electrolyte system for electrochemical glycosylation; 2 + 1+1 strategy enables us to access the tetrasaccharide precursor and complete the synthesis Myc-IV(C16:0, S) efficiently.

Fast and Low-Cost Purification Strategy for Oligosaccharide Synthesis Based on a Hop-On/Off Carrier

A feasible and convenient strategy for oligosaccharide synthesis, which realizes reaction in solution while product purification occurs only by solid-liquid filtration, has been developed. By using a hop-on/off carrier (polytetrafluoroethylene particle), rapid synthesis of tumor-associated antigen Globo-H hexasaccharide has been successfully achieved within 5 steps in 48% overall yield without any intermediate purification by column chromatography. Also, global deprotection, including the cleavage of the tag, proceeded simultaneously only by one-step hydrogenolysis.

HPLC-assisted automated oligosaccharide synthesis: the implementation of the two-way split valve as a mode of complete automation

Reported herein is the development of a user-friendly platform for simple and transformative automation based on standard HPLC equipment. We showcase how the improved platform works in application to the completely automated, a "press of the button," synthesis of various glycan sequences.

Synthesis of defined mono-de-N-acetylated β-(1→6)-N-acetyl-d-glucosamine oligosaccharides to characterize PgaB hydrolase activity

Many clinically-relevant biofilm-forming bacterial strains produce partially de-N-acetylated poly-β-(1→6)-N-acetyl-d-glucosamine (dPNAG) as an exopolysaccharide. In Gram-negative bacteria, the periplasmic protein PgaB is responsible for partial de-N-acetylation of PNAG prior to its export to the extracellular space. In addition to de-N-acetylase activity found in the N-terminal domain, PgaB contains a C-terminal hydrolase domain that can disrupt dPNAG-dependent biofilms and hydrolyzes dPNAG but not fully acetylated PNAG. The role of this C-terminal domain in biofilm formation has yet to be determined in vivo. Further characterization of the enzyme's hydrolase activity has been hampered by a lack of specific dPNAG oligosaccharides. Here, we report the synthesis of a defined mono de-N-acetylated dPNAG penta- and hepta-saccharide. Using mass spectrometry analysis and a fluorescence-based thin-layer chromatography (TLC) assay, we found that our defined dPNAG oligosaccharides are hydrolase substrates. In addition to the expected cleavage site, two residues to the reducing side of the de-N-acetylated residue, minor cleavage products on the non-reducing side of the de-N-acetylation site were observed. These findings provide quantitative data to support how PNAG is processed in Gram-negative bacteria.

Leloir Glycosyltransferases in Applied Biocatalysis: A Multidisciplinary Approach

Enzymes are nature’s catalyst of choice for the highly selective and efficient coupling of carbohydrates. Enzymatic sugar coupling is a competitive technology for industrial glycosylation reactions, since chemical synthetic routes require extensive use of laborious protection group manipulations and often lack regio- and stereoselectivity. The application of Leloir glycosyltransferases has received considerable attention in recent years and offers excellent control over the reactivity and selectivity of glycosylation reactions with unprotected carbohydrates, paving the way for previously inaccessible synthetic routes. The development of nucleotide recycling cascades has allowed for the efficient production and reuse of nucleotide sugar donors in robust one-pot multi-enzyme glycosylation cascades. In this way, large glycans and glycoconjugates with complex stereochemistry can be constructed. With recent advances, LeLoir glycosyltransferases are close to being applied industrially in multi-enzyme, programmable cascade glycosylations.

Differential Recognition of Deacetylated PNAG Oligosaccharides by a Biofilm Degrading Glycosidase

Exopolysaccharides consisting of partially de-N-acetylated poly-β-d-(1→6)-N-acetyl-glucosamine (dPNAG) are key structural components of the biofilm extracellular polymeric substance of both Gram-positive and Gram-negative human pathogens. De-N-acetylation is required for the proper assembly and function of dPNAG in biofilm development suggesting that different patterns of deacetylation may be preferentially recognized by proteins that interact with dPNAG, such as Dispersin B (DspB). The enzymatic degradation of dPNAG by the Aggregatibacter actinomycetemcomitans native β-hexosaminidase enzyme DspB plays a role in biofilm dispersal. To test the role of substrate de-N-acetylation on substrate recognition by DspB, we applied an efficient preactivation-based one-pot glycosylation approach to prepare a panel of dPNAG trisaccharide analogs with defined acetylation patterns. These analogs served as effective DspB substrates, and the rate of hydrolysis was dependent on the specific substrate de-N-acetylation pattern, with glucosamine (GlcN) located +2 from the site of cleavage being preferentially hydrolyzed. The product distributions support a primarily exoglycosidic cleavage activity following a substrate assisted cleavage mechanism, with the exception of substrates containing a nonreducing GlcN that were cleaved endo leading to the exclusive formation of a nonreducing disaccharide product. These observations provide critical insight into the substrate specificity of dPNAG specific glycosidase that can help guide their design as biocatalysts.

Electrochemical Glycosylation as an Enabling Tool for the Stereoselective Synthesis of Cyclic Oligosaccharides

Electrochemical glycosylation of a linear oligosaccharide with a protecting-group-free primary hydroxyl group afforded cyclic oligo-saccharides, up to hexasaccharides, in high yields. Precursors of the cyclic oligosaccharides were prepared by automated electro-chemical assembly-a method for the automated electrochemical solution-phase synthesis of oligosaccharides. We demonstrated that electrochemical glycosylation is useful not only for intermolecular glycosylation but also for intramolecular glycosylation to synthesize cyclic oligosaccharides.

Acid-mediated N-iodosuccinimide-based thioglycoside activation for the automated solution-phase synthesis of α-1,2-linked-rhamnopyranosides

Carbohydrate structures are often complex. Unfortunately, synthesis of the range of sugar combinations precludes the use of a single coupling protocol or set of reagents. Adapting known, reliable bench-chemistry reactions to work via automation will help forward the goal of synthesizing a broad range of glycans. Herein, the preparation of di- and tri-saccharides of alpha 1→2 rhamnan fragments is demonstrated using thioglycoside donors with the development for a solution-phase-based automation platform of commonly used activation conditions using N-iodosuccinimide (NIS) with trimethylsilyl triflate. Byproducts of the glycosylation reaction are shown to be compatible with hydrazine-based deprotection conditions, lending broader functionality to this method as only one fluorous-solid-phase extraction step per coupling/deprotection cycle is required.

Manual and Automated Syntheses of the N-Linked Glycoprotein Core Glycans

Presented herein are two complementary approaches to the synthesis of the core N-glycan pentasaccharide. The first, a traditional manual approach in solution, makes use of the H-bond-mediated aglycone delivery method for the highly diastereoselective introduction of the β-mannosidic linkage at room temperature. The synthesis of the core pentasaccharide was also accomplished using an high-performance liquid chromatography-assisted automated approach. The overall assembly was swift (8 h) and efficient (31%).

Automated Glycan Assembly: A Perspective

The intrinsic complexity of carbohydrate structures has hampered access to pure glycans and hence impeded progress in the glycosciences. Automated Glycan Assembly (AGA) has facilitated the procurement of synthetic glycans, to be used in diagnostics, vaccine development, enzyme characterization and structure-function relationship studies. A general approach for obtaining complex glycans from mammalian, bacterial, fungal and plant classes provides molecular tools for glycobiology research. Recent advances in AGA technology pave the way for the production of novel carbohydrate materials. This perspective describes the state-of-the art of AGA and aspects of the technology where additional improvements are needed.

Machine-Driven Enzymatic Oligosaccharide Synthesis by Using a Peptide Synthesizer

For decades, researchers have endeavored to develop a general automated system to synthesize oligosaccharides that is comparable to the preparation of oligonucleotides and oligopeptides by commercially available machines. Inspired by the success of automated oligosaccharide synthesis through chemical glycosylation, a fully automated system is reported for oligosaccharides synthesis through enzymatic glycosylation in aqueous solution. The designed system is based on the use of a thermosensitive polymer and a commercially available peptide synthesizer. This study represents a proof-of-concept demonstration that the enzymatic synthesis of oligosaccharides can be achieved in an automated manner using a commercially available peptide synthesizer.

Automated Chemical Oligosaccharide Synthesis: Novel Approach to Traditional Challenges

Advances in carbohydrate chemistry have certainly made common oligosaccharides much more accessible. However, many current methods still rely heavily upon specialized knowledge of carbohydrate chemistry. The application of automated technologies to chemical and life science applications such as genomics and proteomics represents a vibrant field. These automated technologies also present opportunities for their application to organic synthesis, including that of the synthesis of oligosaccharides. However, application of automated methods to the synthesis of carbohydrates is an underdeveloped area as compared to other classes of biomolecules. The overarching goal of this review article is to present the advances that have been made at the interface of carbohydrate chemistry and automated technology.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.